[ticket/11548] Check upload avatar URL the same way as in phpBB 3.0

The upload avatar URL was checked for its length in phpBB 3.0. Additionally,
starting with the new avatar system in phpBB 3.1, the URL was checked to
prevent improper URLs being submitted. This minor change is needed for proper
testing of the ucp and acp groups pages.

PHPBB3-11548
[ticket/11548] Fix test errors in groups test on develop

This will fix the errors that appeared in the functional groups test due to
the changes in the new avatar system. The patch will make them work again.

PHPBB3-11548
Merge remote-tracking branch 'remotes/Marc/ticket/11548-develop' into develop

# By Marc Alexander
# Via Marc Alexander
* remotes/Marc/ticket/11548-develop:
  [ticket/11548] Fix test errors in groups test on develop
  [ticket/11548] Check upload avatar URL the same way as in phpBB 3.0